Small unmanned aircraft systems (UASs) typically navigate using GPS and fly at a nominal altitude to avoid obstructions on the earth surface. When GPS is not available or intentionally denied, a UAS may not be able to accurately navigate to its destination as its inertial navigation system (INS) may drift. Visual flight references can sometimes be used to remove INS errors. However, there are several deficiencies. For example, the fact that a UAS has to fly at relatively high altitudes to provide a visual sample space of sufficient geographic coverage to locate its position makes the UAS more vulnerable to detection. Further, a large image database has to be either carried on-board, which impacts platform endurance, or a data-link is required to reach back to an off-board processor. Accessing an off-board processor may not be practical in contested environments. Visual flight references can also be impacted by varied lighting conditions, and are ineffective in degraded visibility environments.
Sometimes synthetic aperture radar (SAR) and passive coherent location (PCL) may be utilized and can provide non real-time data such that the system can build an image of the terrain to its side after it has flown by the area of interest. SAR, nonetheless, requires the sensor to operate at a higher altitude that exposes it to detection. Further, SAR may require substantial on-board computing resources and emits RF energy that may unintentionally reveal the location of the UAS. Passive measurements of RF emissions (e.g., TV and FM radio transmitters) can be used to measure location when RF transmitters are operating and their location(s) are known. PCL requires multiple known RF emitters to triangulate position, and position accuracy may be limited by the type and number of transmitters detected, and by multi-path errors.